1. Field of the Invention
The present invention relates to a method and an apparatus for reproducing an optical signal, and more particularly to a method and an apparatus for reading out an optical signal from a super-resolution optical disk and reproducing information stored in the optical disk.
2. Description of Related Art
In the field of optical recording, high-density recording techniques have been demanded, and recently, a super-resolution optical disk which improves the recording density by using a recording medium which changes the structures of the materials included when light is applied thereto is suggested.
The minimum reproducible mark/pit pitch of an optical disk usually depends on the wavelength λ in the vacuum of light used for reproduction and the numerical aperture NA of an objective lens, and the minimum reproducible mark/pit pitch is usually λ/2NA. A super-resolution optical disk, however, permits reproduction of marks or pits arranged at a pitch less than the usual minimum reproducible mark/pit pitch.
Japanese Patent Laid Open Publication No. 6-183152 discloses that a layer of a phase changing material, such as a Bi alloy, a Te alloy or the like, is formed on a disk with pits thereon and that super-resolution readout is achieved by converging light on the phase changing material layer to turn the phase changing material partly (on the pits) into a liquid phase. Japanese Patent Laid Open Publication No. 5-205314 discloses that a layer of a lanthanoid material is formed on a disk with pits thereon and that super-resolution readout is achieved by changing the reflectance by temperature gradient.
Japanese Patent Laid Open Publication No. 11-250493 discloses a structure wherein a mask layer is provided for a phase changing recording layer made of GeSbTe and wherein an intermediate layer which is made of SiN and which has a thickness of 30 nm is provided between the mask layer and the recording layer. In the structure, when an optical system wherein the wavelength λ is 488 nm and wherein the numerical aperture NA is 0.6 (the length of the minimum reproducible mark is usually 200 nm) is used, even signals from marks with lengths less than 100 nm are detectable.
In the Japan Symposium on Applied Physics 2000, Spring, at presentation No. 28-p-R-2, Kikukawa et al. presented that in a structure wherein Ge, Si, W, etc. are used as reflecting films in a ROM type optical disk with fine pits thereon, when an optical system wherein the wavelength λ is 635 nm and wherein the numerical aperture NA is 0.6 (the length of the minimum reproducible pit is usually 270 nm) was used, a pit with a length of 200 nm could be reproduced. This super-resolution disk is capable of improving the recording density with no particular changes in the optical system. However, the problem is that the CNR (carrier to noise ratio) of small marks or pits is low. Also, Kikukawa et al. found out another problem in a ROM type optical disk with a reflecting film of Ge or the like that when a pit with a length less than the usual minimum reproducible pit length is located between pits with lengths more than the usual minimum reproducible pit length, a signal is partly omitted. This was presented in the International Symposium on Optical Memory 2000, at presentation No. Fr-L-04.